A. Field of the Invention
The present invention relates generally to instruments for measuring the permeability of a material, and, more particularly to calibration of instruments for measuring the permeability of a material.
B. Description of the Related Art
Many products or materials are provided with holes or perforations. Such products and materials require their permeability to be measured. Examples of such products and materials needing permeability measurements include: wallpaper; filters used for air, chemicals, etc.; materials affording the appropriate degree of liquid (ink, varnish, sizing) absorption in printing; porous bags and materials used in food packaging and agricultural fumigation; insulating materials; paper; textiles; etc.
One particular material provided with such holes or perforations are the wrappers of filter cigarettes or similar rod-shaped tobacco products. The perforations allow cool atmospheric air to enter the column of tobacco smoke. Such wrappers are called tipping paper. Running webs of tipping paper making up rod-shaped tobacco products may be perforated mechanically, electrically, or optically. For example, British Patent No. 1,588,980 discloses a perforating unit that employs a set of needles or analogous mechanical perforating tools that puncture selected portions of the running web. U.S. Pat. No. 2,528,158 and British Patent No. 1,604,467 disclose electro-perforating tools that employ heat-generating electrodes that combust selected portions of the running web. An optical perforating tool, as disclosed in U.S. Pat. No. 4,265,254, uses coherent radiation from a laser to make perforations of a desired size and with a high degree of reproducibility.
Conventional filter-tipped tobacco products are perforated in the region of their filter plugs to insure that atmospheric air can enter the column of tobacco smoke irrespective of the length of combusted portion of the tobacco-containing section of the product. It is desirable to regulate the permeability of wrappers of all articles of a given tobacco product in such a way that the permeability is consistent or deviates only negligibly from a predetermined value.
Conventionally, closed-loop monitoring systems regulate and control the permeability of rod-shaped tobacco products. Such monitoring systems typically include a sensor(s) or measuring instrument(s) whose feedback is used to adjust the perforating unit if the permeability of the perforated web deviates from an optimum value, as described in U.S. Pat. No. 4,246,775. As set forth in U.S. Pat. No. 4,537,206, two types of sensors are generally used with such monitoring systems to measure permeability: (1) air permeability measuring sensors, and (2) light permeability measuring sensors.
Both air and light permeability measuring instruments need to be periodically calibrated. In general, such instruments are calibrated with certified standards. Conventional air permeability measuring instruments may be statically calibrated using certified standards made of a variety of materials. Calibration of a light permeability measuring instrument must be done dynamically and must use special standards. Current light permeability measuring instruments are calibrated online using a slow-moving perforated tipping paper provided between a light source and a light sensor. This method is inaccurate due to the inherent variability introduced by the tipping paper perforation and due to the residual light penetrating the paper through the non-perforated area. Unfortunately, no calibration standards exist which may be used to calibrate both air and light permeability measuring instruments.
Thus, there is a need in the art to provide a calibration standard that overcomes the problems of the related art and may be used with both air permeability measuring instruments and light permeability measuring instruments.